The present invention relates to semiconductor lasers and detectors for detecting light from a semiconductor laser.
A semiconductor laser is a device which emits light of substantially a single wavelength. The light from this type of laser can be focused to a spot with a diameter comparable to its wavelength. The semiconductor laser belongs to the same family of semiconductor devices as the LED (light emitting diode). However, the light from an LED has a broader spectrum of wavelengths and thus cannot be focused as sharply as a semiconductor laser. The structure and composition of the laser determine its wavelength, its expected lifetime and its light guiding mechanism. Examples of laser structures are gain guided lasers and index guided lasers. Examples of laser structures are shown in U.S. Pat. Nos. 3,479,613, 3,457,468, 3,293,513, 3,257,626, and 4,483,480.
Most commercial semiconductor lasers contain a photodetector behind the lasing structure to monitor the level of light emitted at the front of the laser chip. The monitoring photodetector provides a signal which is used to maintain a constant laser output. It has been noted in some applications that when a small amount of light emitted from the laser is fed back into the laser chip, it causes a change in the laser output power which can be detected by the rear photodetector. Otherwise, most applications use a separate photodetector to monitor light reflected off of an object by a laser. This light is typically supplied to the separate photodetector through a beam splitter or other device.
FIGS. 1 and 2 show front and side views, respectively, of a typical prior art laser. A semiconductor laser diode chip 12 is mounted on a heat sink 14. A wire 16 is coupled to the top of the semiconductor chip 12 which forms the anode of the laser diode. Wire 16 is coupled to a pin 18. A wire 20 is coupled to either heat sink 14 or another contact which constitutes the cathode of the laser diode. Wire 20 is coupled to a pin 22. A photodetector 24 is coupled to a pin 26. This provides a three pin laser diode which is typically packaged in a standard transistor TO5 can. Photodetector 24 is slightly tilted as shown in the side view of FIG. 2. This tilt of photodetector 24 is provided to prevent light which is reflected off of the photodetector from being fed back into laser chip 12.